Conventional distributed acoustic sensing (DAS) systems rely on the coupling of energy in propagating seismic waves into longitudinal vibrational modes of the fiber (i.e. vibrations of the fiber that are parallel to the axis of the fiber). Typical DAS interrogators send coherent laser pulses into a fiber and measure the Rayleigh backscattered light from those pulses as a function of time (which is then mapped to fiber position). Backscatter from distinct points within the region illuminated by the pulse as it propagates through the fiber interfere and therefore the phase and amplitude of backscatter power received from any given region (corresponding to a pulse width) is very sensitive to the distance between the points in the region where backscatter occurs. Acoustic signals that create longitudinal vibrations in the fiber are detected as variations in the backscattered power from any given region of the fiber as successive laser pulses are sent and the backscatter signals measured as a function of fiber position. Traditional DAS systems are therefore sensitive to any excitations that create vibrations which stretch/compress the fiber along its axis (i.e. longitudinally).
These systems are however insensitive to acoustic energy that is not parallel to the axis of the optical fiber. Methods that allow for determination of acoustic energy in nonlongitudinal orientations to the optical fiber are of interest to those who practice in the art.